VMS Help TCPIP Services, Programming Interfaces, Sockets API, ioctl(), Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The ioctl() function performs a variety of control functions
    on devices. The functions performed are device-specific control
    functions. The request and arg arguments are passed to the file
    designated by fildes and then interpreted by the device driver.
    The basic I/O functions are performed through the read() and
    write() functions.
    Encoded in an ioctl() request is whether the argument is an in
    argument or an out argument, and the size of the arg argument
    in bytes. The macros and definitions used to specify an ioctl()
    request are located in the IOCTL.H header file.